The continuing HIV/AIDS epidemic and the spread of multi-drug resistant Mycobacterium tuberculosis (Mtb) has perpetuated an epidemic of tuberculosis in populations around the world. While BCG is used universally as a vaccine, it is not effective in preventing pulmonary tuberculosis. To combat this ongoing worldwide scourge, vaccine development for tuberculosis is a priority. Convincing experimental evidence exists that CD8+ T cells are required for optimal immunity to tuberculosis in mice and other experimental animals. How T cell mediate protection is incompletely understood, and the old paradigm that IFN-g is end-all and be- all of T cell effector function is being dismantled. Importantly, ther is a huge, clinically important paradox - namely that T cells are crucial to immunity against TB, but the one mediator of protection, IFN-g, does not predict or correlate with protection against disease. Therefore, there must be other correlates of protection. Innovation: We have four retrogenic (Rg) mouse lines, each expressing a different TCR that recognizes the immunodominant epitope TB104-11. Interestingly, these different TCRs vary in their affinity and potential to mediate protection against virulent Mtb. These Rg make feasible experimental strategies to define the molecular basis for how CD8+ T cells mediate protection. Aims: Overview: While there is little doubt that CD8+ T cells make an important contribution to protection against TB, the effector function(s) that inhibit Mtb growth are largely unknown. In Aim 1, we will determine the different effector molecules that contribute to the protection mediated by CD8+ T cells during Mtb infection. Aim 2 will focus on the basic signals that lead to T cell activation and determine how they impact the expression of protective effector functions. These include 1) strength of TCR signaling; 2) the role of CD4+ T cell help and IL-2; and 3) the inflammatory Signal 3. Much of what we know about T cell immunity to Mtb is from studies of primary infection. A more comprehensive understanding of memory T cell responses is needed for vaccine development. We observe a loss of the memory T cells following rechallenge. Thus, an important question is whether vaccine-elicited T cells have a durable phenotype without reversion to ineffectual T cells. Therefore, in Aim 3 we will address whether the primary and secondary CD8+ T cell responses differ in the effector pathways utilized and how they are regulated. Summary: Mtb is a successful pathogen that persists in humans despite a robust immune response. We have three goals: 1) define the mechanisms used by CD8+ T cells to restrict bacterial replication; 2) determine how effector mechanisms are regulated; and 3) determine whether primary and secondary CD8+ T cell responses are regulated differently. Our answers will not only illuminate how T cells mediate protection, but we expect to validate protective pathways that can function as biomarkers to guide vaccines development and evaluation.